1. Field of Invention
The present invention relates to an abnormality detector apparatus for a water cooling type apparatus for cooling an engine. More particularly, the present invention relates to a thermostat abnormality detector apparatus for detecting an abnormality in a thermostat for opening and closing a portion of a cooling water circulation passage.
2. Description of Related Art
A "thermostat" herein refers to an open-close valve in a passage that is switched between the open and closed states at a specific critical temperature. Such a thermostat is used in a cooling water passage of a water cooling-type apparatus.
For example, water cooling-type apparatuses widely employed for automotive engines are designed so that a water pump circulates cooling water between a cooling water passage formed within the engine and a radiator. With this design, the cooling apparatus absorbs heat produced from the interior of the engine and then dissipates the heat from the radiator to the outside.
In order to maintain an appropriate engine temperature, the thermostat opens and closes the cooling water circulation passage. That is, when the cooling water temperature is lower than a predetermined value (normally, about 80.degree. C.), as in the warm-up operation immediately after the engine is started, the thermostat valve closes. When the water temperature becomes higher than the predetermined value, the thermostat valve opens to allow cooling water to circulate between the engine interior and the radiator.
Normally, the thermostat valve closes when the cooling water is at a normal temperature. However, sometimes the thermostat valve may become unable to close from a fully open or semi-open state due to deterioration of a component, for example. If this happens, the engine becomes overcooled and the heater efficiency decreases. Moreover, the overcooling of the engine leads to constant performance of a so-called low-temperature fuel increase control wherein the amount of fuel injected is increased if the engine temperature is low, thereby resulting in deterioration of fuel economy, emissions and the like.
As a countermeasure against this problem, Japanese Patent Application Laid-Open No. 6-213117 (JP 6-213117), for example, proposes a heat insulating and pouring device of cooling water in an automobile wherein a detection device is provided for determining the occurrence of an abnormality in the thermostat if the water temperature detected by a water temperature sensor during a steady engine operation is out of a reference range. The JP 6-213117 application also discloses an arrangement wherein, if the cooling water temperature actually detected by the water temperature sensor at the elapse of a predetermined length of time following a start of the engine, is lower than a predetermined value, it is determined that there is an abnormality in the operation of the thermostat.
In a control device of an automatic transmission described in Japanese Patent Application Laid-Open No. 7-12217, the amount of change in the cooling water temperature for each predetermined length of time is calculated on the basis of a detection value provided by a water temperature sensor. An abnormality in a thermostat due to deterioration of a component or part thereof is detected on the basis of an abnormal change for example, a sluggish temperature change in the cooling water temperature.
However, the former arrangement performs abnormality determination based solely on water temperature and is not provided with any measure for discriminating states of the engine or various operating conditions. Therefore, the aforementioned conventional arrangement cannot achieve sufficiently high reliability in abnormality determination.
Although the latter arrangement calculates the amount of a subtle change in the cooling water temperature on the basis of a detection value provided by the water temperature sensor, the detection signal from the water temperature sensor is insufficient in terms of the following or tracing characteristic and precision for subtle temperature changes.
The manner of change in the cooling water temperature considerably varies during an initial period after a start of the engine, depending on the engine operating state, external environment conditions, and the like, leaving aside the operating state of the thermostat. Therefore, the latter arrangement incurs a significant possibility of false abnormality determination, thereby failing to achieve sufficiently high reliability in abnormality determination. For example, the arrangement may determine that there is an abnormality in the thermostat even through the thermostat is actually operating normally.
Additionally, if an abnormality in the thermostat is determined solely based on an abnormality represented by a change in the detection signal from the water temperature sensor, the arrangement may incorrectly determining that the thermostat has an abnormality when the water temperature sensor has a partial disconnection, contact failure and the like. In many cases in reality, the detection signal changes as if there were an abnormality in the thermostat. The aforementioned conventional art has another drawback that it is always necessary to wait for the elapse of a pre-set length of time before it can be confirmed that the operation of the thermostat is normal.